A group III nitride semiconductor including an aluminum gallium nitride layer (AlxGa1-xN, 0≤x≤1) has a band structure with direct band gap at ultra violet range corresponding to the wavelength of 200 nm to 360 nm, hence a highly efficient ultraviolet device can be produced. Such group III nitride semiconductor device is produced by carrying out the crystal growth of the group III nitride semiconductor thin film on the single crystal substrate by vapor phase deposition methods such as Metal Organic Chemical Vapor deposition (MOCVD) method, Molecular Beam Epitaxy (MBE) method, or Hydride Vapor Phase Epitaxy (HVPE) method or so. Among these, MOCVD method is most widely used industrially, because the thickness of the film can be controlled at atomic layer level, and also relatively high growth rate can be obtained.
Also, as the single crystal substrate allowing the crystal growth of the group III nitride semiconductor thin film, for example the aluminum nitride single crystal substrate obtained by known crystal growth methods such as HVPE method disclosed in the patent document 1 and a Physical Vapor Transport method are used. The aluminum nitride single crystal substrate obtained as such is usually processed into ultraflat surface by carrying out Chemical Mechanical Planarization or so which uses an abrasive such as colloidal silica or so against a surface of said substrate. As such, by making the substrates surface ultraflat, the aluminum gallium nitride layer (AlxGa1-xN, 0≤x≤1) can be easily deposited on said substrate, and also those with high quality can be obtained.
Also, the patent document 2 discloses a cleaner composition for electronic device substrate which includes alkaline metal hydroxides and citric acid. According to the composition disclosed in this article, the abrasive and polishing debris remaining on the substrate surface can be efficiently removed when polishing the electronic device substrate. The electronic device substrate which is the subject of the cleaning in this article is the semiconductor substrate or so having an alkaline resistance, and it discloses that silicon, silicon carbide, zinc oxide and sapphire or so are preferable.